Hydrocracking refers to a process in which hydrocarbons crack in the presence of hydrogen and catalyst to lower molecular weight hydrocarbons. Depending on the desired output, the hydrocracking zone may contain one or more beds of the same or different catalyst. Hydrocracking is a process used to crack hydrocarbon feeds such as vacuum gas oil (VGO) to diesel including kerosene and gasoline motor fuels.
Mild hydrocracking is generally used upstream of a fluid catalytic cracking (FCC) or other process unit to improve the quality of an unconverted oil that can be fed to the downstream FCC unit, while converting part of the feed to lighter products such as diesel. As world demand for diesel motor fuel is growing relative to gasoline motor fuel, mild hydrocracking is being considered for biasing the product slate in favor of diesel at the expense of gasoline. Mild hydrocracking may be operated at a lower severity than partial or full conversion hydrocracking to balance production of diesel with the FCC unit, which primarily is used to make naphtha. Partial or full conversion hydrocracking is used to produce diesel with less yield of the unconverted oil which can be fed to a downstream unit.
Due to environmental concerns and newly enacted rules and regulations, saleable diesel must meet lower and lower limits on contaminants, such as sulfur and nitrogen. New regulations require essentially complete removal of sulfur from diesel. For example, the ultra low sulfur diesel (ULSD) requirement is typically less than about 10 wppm sulfur.
The cetane rating of diesel can be improved by saturating aromatic rings. Catalysts for saturating aromatic rings are typically noble metal catalysts. The cloud point and pour point of diesel can be improved by isomerizing paraffins to increase the degree of branched alkyl groups on the paraffins. Isomerization catalysts can also be noble metal catalyst. Noble metal catalysts are typically poisoned by sulfur species.
There is a continuing need, therefore, for improved methods of producing more diesel from hydrocarbon feedstocks than gasoline. Such methods must ensure that the diesel product meets increasingly stringent product requirements.